Photoelectric smoke detection system



April 7, 1942. F. c. EVANS ET AL- PHOTOELECTRIC SMOKE DETECTION SYSTEMFiled MarCh 15, 1959 [DETECTOR COMPENSATOR INVENTOR F. c. EVANS K.O.DONELIAN BY M 'ATTORNEY Patented Apr. 7, 19.42

Francis 0. Evans, Dongan Hills, and Khatchik 0.

Donelian, New York, N. Y., assignors to American District TelegraphCompany, Jersey City,

I N. J., a corporation of New Jersey Application March 15, 1939, SerialN0. 261,878

4 Claims.

This invention relates generally to light-sensitive detecting or controlsystems adapted primarily for purposes of operating alarm devices orelec- The intrical release devices of various types. vention isparticularly adapted for use as a fire detection system for closingdampers and stopping fans in air conditioning or ventilating systemswherein it is necessary to detect fire conditions when existing eitherin the air conditioning system or in or near the premises served by suchsystem.

Smoke detecting systems used for fire detection must be unusuallysensitive and also reliable to the extent that ordinary fluctuations inthe commercial supply voltage will not cause false alarms or undesiredcontrol of the air conditioning system. The smoke detector must also beunder constant supervision from a central control point whereby troubleconditions may be discovered promptly to prevent accidental periods of'inoperativeness which might allow fires to occur without causing analarm. -In prior smoke detectors of the photoelectric type, it has beennecessary to utilize complicated and expensive apparatus for the purposeof compensating for variations in supply voltages. This inventionprovides an extremely sensitive and simple circult wherein the abovementioned requirements are met.

a The primary object of this invention is to pro-' vide a photoelectricsmoke detecting system of high sensitivity wherein the sensitivityismaintained over a high range of voltage fluctuations in the electricalsupply circuit and wherein constant trouble supervision is maintained.

, 'Another object of this invention is to provide a photoelectricsmoke-detecting circuit which prevents false alarms or false operationsof load circuits due to voltage fluctuations.

Still another object of this invention is to provide a light-sensitivecircuit wherein trouble con- 1 ditions maybe supervised from a remotepoint.

A further object of this invention is to provide indications of troublein the amplifier associated with the photoelectric smoke detectingcircuit.

A still. further object of this invention is to provide indications oftrouble in the photoelectric cell branch of the smoke detecting system.

Still another object of this invention is to provide a supervisorycircuit for indicating trouble conditions within the light source whichis used in connection with a smoke detecting device or failure of thecircuit of said light source.

A further object of this invention is to provide a supervisory circuitcomprising a single trouble responsive element for detecting troubles incertain component parts of a smoke detecting system.

Another object of this invention is to provide 5 an amplifier andassociated photoelectric cell circuit which is suitable for supervisionby a single trouble responsive unit.

Still a further object of this invention is to v provide a smokedetection circuit which can be supervised by a single trouble responsiveelement.

Still another object of this invention is to provide a new and improvedsensitive smoke detecting circuit wherein fluctuations in supply voltagemay be compensated, alarm conditions and trouble conditions aredistinguished from one another "and which is simple in arrangement.

Further objects and advantages of this invention will be evident fromconsideration of the iollowing'description of. the preferred embodimentshown in the accompanying drawing wherem. v The figure oi the drawingshows a circuit diagram of the smoke detector system embodying I thisinvention In order to utilize photoelectric smoke detecting systems foralarm purposes they must be highly sensitive or responsive to smallvariations of received light radiation over a wide range of voltagefluctuations in the electrical supply cirrm cultand they must also bearranged for constant supervision whereby indications of trouble may betransmitted to a central control point. The circuit embodying thisinvention and meeting the above requirements cbnsists of a bridgecircuit 3,; containing-the photoelectric cells 2 and 3 which are adaptedto receive light from a light source I. In the preferred application ofthis circuit it is used for smoke detection but it is adapted for use inany application where changes in light 40 may be used for signalling orcontrol purposes.

The cell 2 acts as a detector for detecting smoke as it passes beforethe light source I, while the cell 3 is arranged to receiveunintercepted light from source I, or light unaffected by smoke or otherfactors affecting the cell 2, and acts as a compensator for compensatingfluctuations in supply voltage. The cells are connected in a bridgecircuit with the condensers 8 and 8 as shown in the drawing. The bridge.comprising the cells 2 and 3 and condensers 6 and 8 is normally in abalanced condition whereby changes in supply voltage affecting theamount of light emitted by the light source I do not disturbthebalanced-condition and consequently no change in output occurs.

tube in which is in turn connected to an amplifler id. The normal presetoutput of this amplifier l4 controls the alarm relay 28 and the troublerelay 2! whereby changes in amplifier output result either in alarmsignals or trouble signals. I

Since under normal conditions light from the source 5 ialls on both thecells 2 and 3, they are conductive and the potentials occurring acrosseach cell will be determined by the light radiations received by each.Consequently the grid ii of the amplifier it will be forced to assumethe potential occurring at junction point 50 of the two photocells. Thebias voltage on grid 12 is provided by means of potentiometer 25 and isso adjusted as to, maintain the circuit in its most sensitive condition.Should smoke or any other suspended matter reduce the amount of lightfalling on cell 2, the conductivity of the detecting photocell 2 will bedecreased thus increasing the impedance of that photocell and drivingthe grid it to a more negative bias with a consequent reduction in. theplate output of tube it). Inspection of the circuit diagram will showthat the second stage it of the amplifier is operated backto-back withrespect to the first stage it) and that,

the output from amplifier I6 is used directly for controlling thecontrol grid it of tube 54. Hence the decrease in the plateoutput in thetube it due to an interruption of the light beam from source 3 causesthe grid to to be driven to a more positive bias on the next half cycleand increases the current output from plate 1? very sharply. Alarm relay2c is adjustedto become energized on plate currents above the normalpreset output of plate ll whereby the increase in output resulting frominterception of the light beam from source l causes relay 2!! toenergize and close its contacts to transmit an alarm signal to alarm 22.

As mentioned above, the voltage fluctuations in the electrical supplycircuit are balanced out in .the bias of grid l2 becomes more positive,resulting in a more negative bias in tube 14 and a decrease in theoutput from plate I! whereby relay 2| becomes deenerglzed to transmit atrouble alarm. ,Obviously it either of the tubes ID or 14 should faildue to filament burnout, the energizing current of relay 2! would ceaseand a trouble condition would result in energization of troubleindicator 24. This operation results from the fact that the filaments oftubes ill and it are connected in series with the voltage source wherebyfailure of the filament in tube it opens the series circuit andextinguishes the filament of tube 14 whereby electron emission thereinwill cease. The same is true when the filament of tube H fails.

From the above description it is evident that this invention provides ahighly sensitive smoke detecting arrangement which maintains itssensitivity over a wide range of voltage fluctuation in the supplycircuit. It is further evident that a single trouble relay is enabled tosupervise component parts of the smoke detector. circuit whereby troublealarms will result upon a failure or trouble condition in the circuit.

Various modifications may be made in the system embodying this inventionwithout departing from the spirit and scope thereof and we desiretherefore that only such limitations shall be placed thereon as areimposed by the prior art and as set forth in the appended claims.

We claim:

1. In a photoelectric circuit, a source of light, a

' balanced photoelectric circuit responsive to vari- 1 ations of lightfrom said source comprising a compensator cell arranged to receiveuninter rupted light from said source, a detector cell connected inseries with the compensator cell and arranged to receive light from saidsource and.

detect the passage of foreign matter across a supervised area betweensaid source and cell, a pair or condensers connected in series acrosssaid cells for limiting the impedance increase in said balanced circuitwhen the light source fails, and

a connection Irom a common point between said the same direction and inproportion, thereby keeping the bridge circuit which is connected to thegrid E2 of amplifier lil'in a balanced condition. When both photocellsare dark, the condensers 8 and 8 connected in parallel with thephotocells 2 and 3 maintain the grid l2 of the tube H3 at a preset biasor potential which is slightly positive with respect to the normal biasor potential which exists when both the photocells are illuminated.These condensers are necessary due to the fact that the impedance of thephotocells in their dark condition approaches infinity and hence if thegrid were not stabilized at a given potential as described above itmight assume such a potential that unstable operation of the amplifierwould result. The importance of this bias adjustment will appear laterin connection with trouble supervision. As a result of the bridgearrangement,'the smoke detector circuit is highly sensitive over a widerange of voltage fluctuations. age may actually vary as much as 20%without any perceptible change in the sensitivity of the system.

The relay 2 I is provided i'or transmitting trouble indications and isnormally energized by a preset value of current during normal conditionscondensers to a common point between said cells whereby each condensershunts a cell, resistance means connected across said balanced circuit,a

source of alternating potential connected across said light source andsaid resistance means whereby the potential at said common points isnormally constant regardless of fluctuations in The fluctuation ofsupply volt the voltage of said source, or said potentialassumes acertain value when the impedance of said detector cell changes or saidpotential assumes an opposite value when said light source fails, avacuum tube switching device connected in series with said source ofpotential, a connection from said common points to the input oi. saidswitching tube for biasing it to decrease its output upon a decrease inlight received by said detector cell or for biasing said tube toincrease its output when said light source tails and for maintainingnormal bias and output in said tube when the light is uninterrupted butfluctuating,

a control vacuum tube connected directly to the output of said switchingtube and in series with said source of potential whereby said controltube is biased for increasing its'outputin response to decreased outputin said switching tube and for decreasing its output in response toincreased output insaid switching tube, alarm means and troubleindication meansconnected in series to 2,278,920 the output of saidcontrol tube, said alarm means ,"a balanced photoelectric circuitresponsive to responding to increases of said control tube outputresulting from detector cell actuation, and said trouble meansresponding to decreases in said control tube output resulting fromfailure of the light source or vacuum tube circuit.

2. In a photoelectric circuit, a source of light, a photoelectriccircuit responsive to variations of light from said source comprising acompensator cell arranged to receive uninterrupted light from saidsource, a detector cell connected in series relation to balance thecompensator cell and arranged to receive light from said source anddetect the passage of foreign matter across a supervised area betweensaid source and cell,

a pair of high impedances connected across said cells, and a connectionfrom a common point between said impedances to a common point betweensaid cells whereby each impedance shunts a cell for limiting theimpedance increase in said balanced circuit when the light source fails,a source of potential connected across said light source and saidphotoelectric circuit whereby the potential at said common points isnormally constant regardless of fluctuations in the voltage of saidsource, orsaid potential assumes a certain value when the impedance ofsaid detector cell changes or said potential assumes an opposite valuewhen said light source fails,-a vacuum tube control device connected inseries with said source of potential, 9. connection from said commonpoints to the input of said control-device for biasing it to increaseits output upon a decrease in light received by said detector cell orfor biasing said control device to decrease its output when said lightsource fails and for maintaining normal bias and output-insaid controldevice when the light is uninterrupted but fluctuating. alarm-means andtrouble indication means connected in series to the output 01 saidcontrol device, said alarm means responding to increases of said controldevice output and said trouble means responding toldecreases in saidcontrol device output.

3. In a photoelectric circuit, a source of light,

variations of light from said source comprising a compensator cellarranged to receive uninterrupted light from said source, a detectorcell connected in series with the compensator cell and arranged toreceive light i'romsaid source and detect the passage of foreign matteracross a supervised area between said source and cell,

high impedance means connected across said cells, and a commonconnection between each of said impedance means and said cells, a sourceof potential connected tosaid light source and said balanced circuitwhereby the potential at said common connection is normally constantregardless of fluctuations in the voltage of said source, or saidpotential assumes a certain value when the impedance of said detectorcell changes or said potential assumes an opposite value when said lightsource fails, alarm means and trouble indication means controlled by thpotential of said common connection, said alarm means responding whenchange oi received light changes the impedance of the detector cell andsaid trouble means responding to voltage changes when the light sourcetails.

4. In a photoelectric circuit, a source of light, a plurality ofphotoelectric cells connected in series balanced relation, one of saidcells arranged to receive uninterrupted light from said source and actas a compensator cell, the other of said cells arranged to detectvariations in light received from said source across a supervised areawhereby normal fluctuation of the light source has no eflect. a sourceof potential connected to the balanced circuit, high impedance means inparallel with each cell for limiting the impedance increase in saidbalanced circuit when said light source fails, alarm means responsive tosaid balanced circuit when light to said detector cell is obstructed andtrouble indieating means responsive to voltage changes caused b thelimited impedance increase when,

